[填空题]Technological invention has directly or indirectly produced social change in every period of history. The invention of the horse collar in the Middle Ages was a{{U}} (36) {{/U}}technological advance for agriculture; the{{U}} (37) {{/U}}for use at sea so{{U}} (38) {{/U}}navigation in the fifteenth century that it led in the age of{{U}} (39) {{/U}}; and the steam engine was an {{U}} (40) {{/U}}factor in the industrialization of Europe in the nineteenth century. A discovery is another kind of technological{{U}} (41) {{/U}}. While inventions add something new to a culture, discoveries increase our knowledge of something that already exists. Gravity had always been there, but Newton "discovered" it: be{{U}} (42) {{/U}}how this natural force worked and how its laws{{U}} (43) {{/U}}to falling bodies. This knowledge made possible many new inventions. {{U}} (44) {{/U}}. Since each invention makes more inventions possible, the speed of technological development tends